The Stovebolt Site Forums The Shop Area Engine Shop Help with crankcase vent

I have a 1961 235. Took the draft tube and plumbing apart a year ago. Maybe I forgot what I thought I would not forget. I need help to be sure I have it vented correctly with the type components I have. I have a non-road draft tube with a side bung fitting. Then I believe a PCV is installed there and then a hose to base of intake. I think I do not have a vented oil cap in valve cover? and the valve cover is not vented. First question: Does that sound OK plumbing? Second question: PCV closes in the direction of the intake. It seems like that the vacuum from intake manifold would keep valve closed or is it designed to stay open only at idle? or is it backwards? I have been running a little bit with both ends plugged and it runs good but afraid I am going to cause a leak or worse. Ready to add hose and etc. when I hear from those in the know.

PCV sounds backwards, intake should suck all the time to keep lower fumes moving but close in case of backfire or such - and you do need a vented oil fill cap so the air getting sucked outta the crankcase is replaced, otherwise vacuum in there will be sucking blowby

Bill

I'm thinking you should have a vented oil cap. At first they made what the called an open system, that drew air in the oil cap vent and the fumes were drawn into the intake to be burned.
Later cars and trucks had a closed system that had a hose from the air cleaner that was plumbed into the valve cover and the PVC was also fitted to the valve cover back to the intake manifold.

John

The more I read and think the more confused I get. The supposed reason for venting is two fold. Get rid of pollution and relieve pressure cause by "normal" blowby into crankcase. So the built up pressure is from partial blowby from ignition/combustion. Then I'm thinking on my setup, where am I getting air intake if I have no valve cover breathing? Then I think, who cares about the valve cover area as long as I'm sucking pressure out of crankcase? If the PVC closes under increased manifold vac then I'm no longer sucking blowby?

a PCV was available on the TF series, see the shop manual - the valve has a spring in it that prevents it from opening until enough vacuum occurs in the manifold, vacuum opens it

Bill

This diagram shows direction of air/gas flow in the 1954 PCV system. The oil filler cap is non-vented. This design dates back to its domestic truck introduction in 1948.

Even into the 60s, the Closed Ventilation System was still in use for California vehicles. No PVC valve, no vented fill cap. Tubes connected from rocker cover near the filler to the air cleaner, and through a metered orifice from the rear of the rocker cover to the intake manifold.

Thanks Guys. I think I have to re-examine what I have. I don't have that air cleaner, plain oil bath, no air tube. I think I will plug hole at manifold and vent breather to air. I found a pic I took of it in stock condition and the PCV closes upon obtaining increased vacuum. It's the only way it can be screwed into system. So it's possible PO did something wrong and is misleading me. Web said first use of PCV was 61/62, looks like that was wrong.

For the AD mod, its simple valve where the road draft tube used to be, connected to the intakes vac port. The other really important part is a vented oil cap. Done and done. Air flows from the vented cap to the draft tube pcv to the intake. Very balanced system that keeps the gasses out and clean air in. All without oil dripping all over your driveway (via road draft tube). Hope this helps.

Not sure I am understanding your issue Ken, but the intake suction pulls the valve OPEN, not shut. Would that make more sense?

Tims manual page link shows without a vented oil fill cap it should have the hose from valve cover to air cleaner to get air in

Bill

The PVC valve must prevent backflow so that a backfire will not ignite the crankcase gases (a great way to loose all your gaskets). Under normal flow, the PVC valve actually has a calibrated flow curve. Max flow is desired under heavy loads, where blowby gases are high. At low loads or idle, there is little blowby to worry about, hence the lower flow of the valve. This is also why it is important to use the correct PCV valve for your engine.

Yes makes sense that it should open with vac applied. I will make sure I my looking at/testing it properly to be sure it's the wrong PCV. Looks like I have 61/62 ,either car or truck,
RPO242
In my earlier info I implied that they first came out in 1961. Actually that was the year GM provided them on ALL California cars and then nationally in 1962. According to unnamed historians. I got that RPO number from a copy of a "parts book" reprint 1973. So looks like PCV (or PVV as Bill and TC's info indicates) was possibly an option way back, at least in 61/62 it was RPO242.

If I put the PCV I have on the opposite end of the system (screwed into manifold instead of tube) it would reverse it's closure action. That's not where it was when I got the truck. It's not where they are placed on all info I've seen. Then I will put on a big old air breathing oil cap. Am I getting there? Am I normal?

Anybody have a twist on vent cap part number, that fits a 235 cover? can only find clip on type.

factory PCV on an early 283, which is really the same setup as the 6 cyl

Bill

http://www.speedprint.com/deves50/pcvinstall.php

I wrote that many years ago and it has proven to work really well for myself and others. At first my main concern was sucking oil into the intake, but turns out that isn't an issue at all. I was going to move the valve further away from the road draft, but it fits so nicely there, so I just went with it. I get lots of positive comments.

I added a PCV to my 216 and it's worked out pretty good. My 216 has the 3 vent slots cut in the top of the valve cover but I plugged them up and went w/a vented oil fill cap I got from Classic Truck Parts. It was listed for 53 and up 235s but fits the 216 fine. I originally mounted the PCV per Deve's mod but changed to mounting it into a modified draft tube to cut down on the amount of oil that was getting sucked up into the intake manifold. Altho I indicated the PCV clamp was just temporary I haven't changed it yet
It's simpler to just paste my post from the thread's final setup:

What is PVV?
GM called it PCV in Advance-Design trucks.

What do you mean by a twist on vent cap?
The oil filler cap on the 216/235/261 valve cover?
- vented filler cap
- unvented filler cap

Bill's TF link called the valve: Positive Ventilation Valve, I said PVV. Yes, the system is called Positive Crankcase Ventilation. Bill's pic is the V8 version of RPO 242. First available in 1961 in V8's and 6's.
Sorry for the confusion.
Twist on, vented, valve cover, oil filler cap for 1961 235 cu in as opposed to push on that fits on the front fill tube on V8's, which I find plenty of. I haven't looked very hard yet, just at FLAPS. I will check the vendors.

I noticed same deal, I'm messing with a 59 235 that is stock and it had road draft and unvented valve cover w/solid metal twist in oil fill cap in middle. I'm putting PCV into the spot road draft tube went but haven't found a twist on oil fill that is vented.

correct vented fill caps are available

Bill

Thank you Bill and all.